ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
599 
These have now been shown to occur in extremely young stages of a 
number of animals. This fact gives them a new importance. They 
have too definite a history to allow of their being set aside as mere 
headings or undulations of no metameric significance. At first the 
author attached no particular attention to them either, but a compre- 
hensive study has convinced him of their segmental importance ; and, 
taking everything into consideration, they furnish, he thinks, a more 
satisfactory basis for the interpretation of the metamerism of the head 
than we have had before. It is extremely difficult to represent them 
on paper just as they look. To observe them successfully is largely a 
question of getting shadows. The care with which the specimens have 
been prepared makes a great difference in the clearness with which 
these structures may be seen. The embryos should be 'washed by a very 
gentle jet of the reagent immediately after their immersion, and the 
cloudy reagent should be removed and replaced by clear fluid. 
The author deals closely with the question — are these neural segments 
artifacts ? are they, in other words, produced by the reagents used ? 
The evidence of the care which he has taken seems to show that this 
question must be answered in the negative. They are constant in their 
appearance and general characteristics when different reagents are used. 
They are similar in different kinds of embryos, and they have been seen 
in fresh material before any reagents at all were used. This neuro- 
meric segmentation arises long before there are any segmental divisions 
of the mesoderm, and therefore cannot depend upon segmental divisions 
of the middle germ-layer. Neuromeric segmentation is more primitive 
than mesodermic segmentation. The cells in these segments are charac- 
teristically arranged in the earliest stages, and their arrangement and 
structure appear to indicate that they are definite differentiations of cell 
areas, not merely mechanical undulations. The entire embryo is divided 
into similar segments. In Squalus acanthias there are eleven segments 
in the brain region in front of the vagus nerve, and fourteen paired seg- 
ments in the entire brain region. There is some evidence to show that 
the region of the spinal cord is being encroached upon by backward 
differentiation early in the history of this segmentation. There are seven 
that clearly belong to the hind-brain, and later on two more are added. 
These segments are clearest in the epiblast. The other layers are 
slightly affected by the segmental influence, the mesoblast least of all. 
As to their relation, they are directly related to the cranial and spinal 
nerves. The segments are also directly related to the sense-organs 
through nerves. The modifications are most extreme in the anterior 
part, with the early obliteration of those belonging to the fore* and mid- 
brains. Those in the hind-brain region are clearly defined for some 
time after the establishment of the cranial nerves, and then they fade 
away. The modifications of these segments have not yet been worked 
out in detail. 
Turning to the sense-organs, the author deals first with the lateral 
eyes. Do they, he asks, belong to the same series with the other 
sense-organs, or do they occupy a position by themselves ? While 
owning that this is a puzzling question which has given rise to much 
controversy among anatomists, he thinks there is some reason for 
believing that the eyes are closely related to the other sense-organs, 
and we are now in the attitude of awaiting further facts. After giving 
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